1. Technical Field
The present invention relates to an optical device, a method for manufacturing the optical device, and an optical scanner.
2. Related Art
There is a known optical device employing a structure including a torsional oscillator formed by processing a silicon substrate with the aid of MEMS (micro-electro-mechanical systems) technology (see JP-A-2005-107069, for example). The optical device is used as an optical scanner that deflects light in a printer, a display, or other apparatus.
For example, the optical scanner described in JP-A-2005-107069 includes a reflection mirror having a reflection surface that reflects light incident thereon and elastic beams connected to the reflection mirror. In the optical scanner, the reflection mirror oscillates when the beams deform.
In the optical scanner, when light is incident on the beams or portions other than the reflection surface of the reflection mirror, the light is reflected off the beams and the reflected light forms stray light, which degrades the quality of a resultant image.
To address the problem, for example, the optical scanner described in JP-A-2005-107069 has a non-reflection film made of nickel oxide provided on the entire surface of each of the beams on the side where the light reflection surface is present. The structure including the reflection mirror and the beams is formed by etching a silicon substrate.
When the structure is formed in a dry etching process, irregularities called scallops are disadvantageously formed on the side surfaces of the reflection mirror and the beams. On the other hand, when the structure is formed in a wet etching process, corners resulting from the silicon crystal plane are disadvantageously formed along the interface between the reflection mirror and the beams.
When such irregularities and corners are formed on the beams, stress concentration tends to occur when the reflection mirror oscillates, resulting in decrease in lifetime of the optical scanner. It is therefore necessary to planarize the irregular side surfaces and round the corners. To this end, it is effective to perform a thermal treatment using silicon surface diffusion motion.
This approach, however, involves the step of forming the non-reflection film and the step of planarizing the irregular side surfaces and rounding the corners to prevent stress concentration, disadvantageously resulting in increase in the number of steps and decrease in productivity.